U.S. patent application number 15/698331 was filed with the patent office on 2018-03-08 for dielectric barrier discharge ionization detector.
This patent application is currently assigned to Shimadzu Corporation. The applicant listed for this patent is Osaka University, Shimadzu Corporation. Invention is credited to Katsuhisa KITANO, Kei SHINADA.
Application Number | 20180067082 15/698331 |
Document ID | / |
Family ID | 61280548 |
Filed Date | 2018-03-08 |
United States Patent
Application |
20180067082 |
Kind Code |
A1 |
SHINADA; Kei ; et
al. |
March 8, 2018 |
DIELECTRIC BARRIER DISCHARGE IONIZATION DETECTOR
Abstract
A dielectric barrier discharge ionization detector capable of
achieving a high signal-to-noise ratio is provided. The detector
includes: a discharging section for generating plasma from
argon-containing gas by electric discharge; and a charge-collecting
section for ionizing a component in a sample gas by an effect of
the plasma and for detecting ion current formed by the ionized
component. The discharging section includes a cylindrical
dielectric tube having a high-voltage electrode connected to AC
power source as well as upstream-side and downstream-side ground
electrodes and formed on its outer circumferential wall. A
semiconductor film is formed on the inner circumferential surface
of the tube. The upstream-side and downstream-side ground
electrodes are respectively made longer than the initiation
distances for a creeping discharge between the high-voltage
electrode and a tube-line tip member as well as between the
high-voltage electrode and the charge-collecting section.
Inventors: |
SHINADA; Kei; (Kyoto,
JP) ; KITANO; Katsuhisa; (Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shimadzu Corporation
Osaka University |
Kyoto
Osaka |
|
JP
JP |
|
|
Assignee: |
Shimadzu Corporation
Kyoto
JP
Osaka University
Osaka
JP
|
Family ID: |
61280548 |
Appl. No.: |
15/698331 |
Filed: |
September 7, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 30/64 20130101;
G01N 2030/025 20130101; H05H 2001/2443 20130101; G01N 2030/647
20130101; G01R 19/0061 20130101; G01N 27/66 20130101; G01N 30/54
20130101; G01N 27/70 20130101; H05H 1/2406 20130101 |
International
Class: |
G01N 27/70 20060101
G01N027/70; G01N 30/64 20060101 G01N030/64; H05H 1/24 20060101
H05H001/24 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 8, 2016 |
JP |
2016-175501 |
Claims
1. A dielectric barrier discharge ionization detector for ionizing
and detecting a sample component in a sample gas by using plasma
induced by an electric discharge within a gas passage through which
a plasma generation gas containing argon is passed, the detector
comprising: a) a dielectric tube made of a dielectric material and
containing an upstream section of the gas passage; b) a
high-voltage electrode attached to an outer wall of the dielectric
tube; c) a ground electrode electrically connected to a ground and
arranged so as to face the gas passage, the ground electrode having
a surface which faces the gas passage and is covered with a
dielectric body, with at least a portion of the surface being
located downstream of the high-voltage electrode in a flow
direction of the plasma generation gas; d) an AC power source
connected to the high-voltage electrode, for applying an AC voltage
between the high-voltage electrode and the ground electrode so as
to induce a dielectric barrier discharge within the gas passage and
thereby generate plasma; and e) a charge-collecting section located
downstream of the ground electrode, forming a downstream section of
the gas passage, including a sample-gas introducer for introducing
a sample gas into the downstream section, and a collecting
electrode for collecting ions generated from a sample component in
the sample gas by light emitted from the plasma, wherein: a
semiconductor film is formed on an inner wall or the dielectric
tube over an area where the high-voltage electrode is attached
and/or an area located downstream of the aforementioned area and
upstream of the charge-collecting section; and a length of the
ground electrode in an area located downstream of the high-voltage
electrode is longer than an initiation distance fir a creeping
discharge between the high-voltage electrode and the
charge-collecting section.
2. The dielectric barrier discharge ionization detector according
to claim 1, wherein the semiconductor film is a diamond-like-carbon
film or titanium dioxide film.
3. A dielectric barrier discharge ionization detector for ionizing
and detecting a sample component in a sample gas by using plasma
induced by an electric discharge within a gas passage through which
a plasma generation gas containing argon is passed, the detector
comprising: a) a dielectric tube made of a dielectric material and
containing an upstream section of the gas passage; b) an
electrically grounded tube-line tip member made of metal, for
introducing the plasma generation gas into the dielectric tube; c)
a high-voltage electrode attached to an outer wall of the
dielectric tube; d) an electrically grounded upstream-side ground
electrode attached to the outer wall of the dielectric tube and
located upstream of the high-voltage electrode as well as
downstream of the tube-line tip member in a flow direction of the
plasma generation gas; e) an electrically grounded downstream-side
ground electrode attached to the outer wall of the dielectric tube
and located downstream of the high-voltage electrode in the flow
direction of the plasma generation gas; f) an AC power source
connected to the high-voltage electrode, for applying an AC voltage
between the high-voltage electrode and the upstream-side ground
electrode as well as between the high-voltage electrode and the
downstream-side ground electrode so as to induce a dielectric,
barrier discharge within the gas passage and thereby generate
plasma; and g) a charge-collecting section located downstream of
the downstream-side ground electrode, forming a downstream section
of the gas passage, including a sample-gas introducer for
introducing a sample gas into the downstream section, and a
collecting electrode for collecting ions generated from a sample
component in the sample gas by light emitted from the plasma,
wherein: a semiconductor film is formed on an inner wall of the
dielectric tube over an area where the high-voltage electrode is
attached and/or an area located downstream of the aforementioned
area and upstream of the charge-collecting section; and a length of
the downstream-side ground electrode in the flow direction is
longer than an initiation distance for a creeping discharge between
the high-voltage electrode and the charge-collecting section.
4. The dielectric barrier discharge ionization detector according
to claim 3, wherein the semiconductor film is a diamond-like-carbon
film or titanium dioxide film.
5. A dielectric barrier discharge ionization detector for ionizing
and detecting a sample component in a sample gas by using plasma
induced by an electric discharge within a gas passage through which
a plasma generation gas containing argon is passed, the detector
comprising: a) a dielectric tube made of a dielectric material and
containing an upstream section of the gas passage; b) an
electrically grounded tube-line tip member made of metal, for
introducing the plasma generation gas into the dielectric tube; c)
a high-voltage electrode attached to an outer wall of the
dielectric tube; d) an electrically grounded upstream side ground
electrode attached to the outer wall of the dielectric tube and
located upstream of the high-voltage electrode as well as
downstream of the tube-line tip member in a flow direction of the
plasma generation gas; e) an electrically grounded downstream-side
ground electrode attached to the outer wall of the dielectric tube
and located downstream of the high-voltage electrode in the flow
direction of the plasma generation gas; f) an AC power source
connected to the high-voltage electrode, for applying an AC voltage
between the high-voltage electrode and the upstream-side ground
electrode as well as between the high-voltage electrode and the
downstream-side ground electrode so as to induce a dielectric
barrier discharge within the gas passage and thereby generate
plasma; and g) a charge-collecting section located downstream of
the downstream-side ground electrode, forming a downstream section
of the gas passage, including a sample-gas introducer for
introducing a sample gas into the downstream section, and a
collecting electrode for collecting ions generated from a sample
component in the sample gas by light emitted from the plasma,
wherein: a semiconductor film is formed on an inner wall of the
dielectric tube over an area where the high-voltage electrode is
attached and/or an area located upstream of the aforementioned area
as well as downstream of the tube-line tip member; and a length of
the upstream-side ground electrode in the flow direction is longer
than an initiation distance for a creeping discharge between the
high-voltage electrode and the tube-line tip member.
6. The dielectric barrier discharge ionization detector according
to claim 5, wherein the semiconductor film is a diamond-like-carbon
film or titanium dioxide film.
Description
TECHNICAL FIELD
[0001] The present invention relates to a dielectric barrier
discharge ionization detector which is primarily suitable as a
detector for a gas chromatograph (GC).
BACKGROUND ART
[0002] In recent years, dielectric barrier discharge ionization
detectors (which are hereinafter abbreviated as the "BIDs")
employing the ionization by dielectric barrier discharge plasma
have been put to practical use as a new type of detector for GC
(for example, see Patent Literatures 1 and 2 as well as Non Patent
Literature 1).
[0003] BIDs described in the aforementioned documents are roughly
composed of a discharging section and a charge-collecting section
which is located below the discharging section. In the discharging
section, a low-frequency AC high voltage is applied to
plasma-generating electrodes circumferentially formed mound a tube
made of a dielectric material, such as quartz glass ("dielectric
tube"), to ionize an inert gas supplied into the tube line of the
dielectric tube and thereby form atmospheric-pressure
non-equilibrium plasma. Due to the effects of the light emitted
from this plasma (vacuum ultraviolet light), excited species and
other elements, the sample components in a sample gas introduced
into the charge-collecting section are ionized. The resulting ions
are collected through a collecting electrode to generate detection
signals corresponding to the amount of ions, i.e. the amount of
sample components.
[0004] FIG. 4 shows the configuration of the discharging section
and surrounding area in the aforementioned BID. As noted earlier,
the discharging section 410 includes a cylindrical dielectric tube
411 made of a dielectric material, such as quartz, the inner space
of which forms a passage of inert gas serving as plasma generation
gas. On the outer wall surface of the cylindrical dielectric tube
411, three ring-shaped metallic electrodes (made of stainless
steel, copper or the like) are circumferentially formed at
predetermined intervals of space. A high AC, excitation voltage
power source 415 for generating a low-frequency high AC voltage is
connected to the central electrode 412 among the three electrodes,
while the electrodes 413 and 414 located above and below the
central electrode are both grounded. Hereinafter, the central
electrode is called the "high-voltage electrode" 412, while the
upper and lower electrodes are called the "ground electrodes" 413
and 414. The three electrodes are collectively referred to as the
plasma generation electrodes. Since the wall surface of the
cylindrical dielectric tube 411 is present between the passage of
the inert gas and the plasma generation electrodes, the dielectric
wall itself functions as a dielectric coating layer which covers
the surface of those electrodes 412, 413 and 414, enabling a
dielectric barrier discharge to occur. With the inert gas flowing
through the cylindrical dielectric tube 411, when the high AC
excitation voltage power source 415 is energized, a low-frequency
high AC voltage is applied between the high-voltage electrode 412
and each of the upper and lower ground electrodes 413 and 414
located above and below. Consequently, an electric discharge occurs
within the area sandwiched between the two ground electrodes 413
and 414. This electric discharge is induced through the dielectric
coating layer (the wall surface of the cylindrical dielectric tube
411), and therefore, is a form of dielectric barrier discharge,
whereby the plasma generation gas flowing through the cylindrical
dielectric tube 411 is ionized over a wide area, forming a cloud of
plasma (atmospheric-pressure non-equilibrium plasma).
[0005] The two ground electrodes 413 and 414 arranged so as to
sandwich the high-voltage electrode 412 in between prevents the
plasma generated by the electric discharge from spreading into the
upstream and downstream sections of the cylindrical dielectric tube
411, whereby the effective plasma generation area is confined to
the space between the two ground electrodes 413 and 414.
[0006] In the BID, the dielectric material which covers the surface
of the plasma generation electrodes in the previously described
manner prevents an emission of thermions or secondary electrons
from the surface of the metallic electrodes. Furthermore, since the
plasma generated by the dielectric barrier discharge is a
non-equilibrium plasma with low-temperature neutral gas, various
factors which cause a fluctuation of the plasma are suppressed,
such as a temperature fluctuation in the discharging section or an
emission of gas from the inner wall of the quartz tube due to the
heat. As a result, the BID can maintain plasma in a stable form and
thereby achieve a higher level of signal-to-noise (SN) ratio than
the flame ionization detector (FID), which is the most commonly
used type of detector for GC.
[0007] In general, there are two types of "dielectric barrier
discharge": an electric discharge generated by a configuration in
which only one of the high-voltage and ground electrodes is covered
with a dielectric body (which is hereinafter called the
"single-side barrier discharge"); and an electric discharge
generated by a configuration in which both of the high-voltage and
ground electrodes are covered with a dielectric body (which is
hereinafter called the "double-side barrier discharge". Non Patent
Literature 1 discloses the result of a study in which two
discharging sections respectively employing those two
configurations were constructed and their detector outputs in a
BID-equivalent structure were compared, which demonstrated that a
higher SN ratio could be achieved with the double-side barrier
discharge than with the single-side barrier discharge.
[0008] As the inert gas fir plasma generation in such a BID, helium
(He) gas or argon (Ar) gas (or He gas with a trace amount of Ar gas
added) is particularly widely used in practice. The reasons for
using those gases are as follows:
[0009] (1) He gas: The discharge light generated by using He gas
has an extremely high energy level of approximately 17.7 eV, making
it possible to ionize and detect the atoms and molecules of most
substances except for neon (Ne) and He. This is particularly useful
fur die detection of inorganic substances, since FIDs cannot ionize
(and therefore cannot detect) inorganic substances.
[0010] (2) Ar gas (or He gas with a trace amount of Ar gas added):
The energy level of discharge light generated by using Ar gas is
approximately 11.7 eV and cannot ionize inorganic substances, as
with the FID. This characteristic is useful in the case of
specifically detecting organic substances. For example, in the case
of detecting a trace amount of organic substance in an aqueous
solution, the trace amount of organic substance of interest can be
easily detected since the water used as the solvent cannot be
detected.
[0011] Since the discharge characteristics vary depending on the
kind of gas, the optimum electrode arrangement (e.g. the width of
each electrode and the spacing of the electrodes) in the
discharging section of the BID also changes depending on whether He
gas or Ar gas is used as the inert gas. Accordingly, BIDs are
configured to allow users to prepare a plurality of cylindrical
dielectric tubes with different electrode arrangements and select a
cylindrical dielectric tube having a suitable electrode arrangement
for the kind of gas to be used. In the following description, a BID
which uses Ar gas (or He gas with a trace amount of Ar gas added)
as the plasma generation gas is called the "Ar-BID". Similarly, a
BID which uses He gas as the plasma generation gas is called the
"He-BID".
[0012] FIG. 5 is a graph obtained by plotting the discharge
initiation voltage far He and Ar at atmospheric pressure against
the inter-electrode distance based on Paschen's law which is an
empirical law concerning the discharge voltage for spark discharge.
As can be seen in the graph, when the inter-electrode distance is
the same, the discharge initiation voltage for Ar is approximately
two times as high as the discharge initiation voltage for He. In
other words, provided that the device should be operated at the
same discharge initiation voltage, the inter-electrode distance for
Ar needs to be equal to or shorter than one half of the distance
for He. Since there are also other parameters affecting the
dielectric barrier discharge employed in BIDs, such as the material
of the dielectric body, gas purity, frequency of the discharge
power source, and waveform of the power source, it is difficult to
predict an optimum electrode arrangement and discharging conditions
from Paschen's law which is the empirical law concerning, spark
discharge. However, from the foregoing discussion, it is at least
possible to conclude that the Ar-BID requires a shorter
inter-electrode distance between the plasma generation electrodes
(or a higher discharge voltage) than the He-BID).
CITATION LIST
Patent Literature
[0013] Patent Literature 1: JP 2010-60354 A
[0014] Patent Literature 2: WO 2012/169419 A
[0015] Patent literature 3: JP 2013-125022 A
Non Patent Literature
[0016] Non Patent Literature 1: Shinada and four other authors,
"Development of New Ionization Detector for Gas Chromatography by
Applying Dielectric Barrier Discharge", Shimadzu Hyouron (Shimadzu
Review), Vol. 69, Nos. 3/4, Mar. 29, 2013
SUMMARY OF INVENTION
Technical Problem
[0017] Due to the previously described reason, the distance between
the neighboring electrodes in conventional Ar-BIDs has been made
shorter than in He-BIDs. However, an Ar-BID configured in this
manner has an evidently lower SN ratio than He-BIDs.
[0018] A study to search for the cause of such a decrease in the SN
ratio in the Ar-BID has experimentally revealed the fact that, in
the case of the electric discharge in Ar gas, although the
inter-electrode distance at which the electric discharge can occur
is short as noted earlier, once the electric discharge is
initiated, the plasma generation area spreads over the entire
cylindrical dielectric tube 411 and eventually reaches the
tube-line tip member 416 provided at the upper end of the
cylindrical dielectric tube 411 as well as the connection member
421 of the charge-collecting section connected to the lower end of
the cylindrical dielectric tube 411. Since the tube-line tip member
416 and the connection member 421 are both made of metal and
electrically grounded, the electric discharge which occurs within
the cylindrical dielectric tube 411 in the aforementioned situation
becomes a single-side barrier discharge between the high-voltage
electrode 412 which is covered with the dielectric body and the
tube-line tip member 416 or connection member 421 which is not
covered with any dielectric body. This is the likely reason why the
SN ratio was lower than in the case of the double-side barrier
discharge.
[0019] The present invention has been developed in view of the
previously described point. Its objective is to prevent the
occurrence of a single-side barrier discharge in the Ar-Bit) and
thereby achieve a high SN ratio.
Solution To Problem
[0020] The present inventors have inferred that the aforementioned
expansion of the plasma generation area beyond the range estimated
by Paschen's law occurs due to the creeping discharge occurring at
the interface between the inner wall of the cylindrical dielectric
tube 411 and the Ar gas. Creeping discharge is a discharge
phenomenon which occurs along the boundary surface between
different dielectrics. In Ar-BIDS, it is likely that this
phenomenon develops from the high-voltage electrode 412 into the
upper and lower areas, to eventually induce a gas discharge between
the high-voltage electrode 412 and the tube-line tip member 416 as
well as between the high-voltage electrode 412 and the
charge-collecting section. That is to say, in the aforementioned
Ar-BID, since the tube-line tip member 416 or the metallic member
(connection member 421) near the upper end of the charge-collecting
section is electrically grounded, a potential gradient is formed
from the high-voltage electrode 412 toward each of those members
416 and 421. If the ground electrodes 413 and 414 provided between
the high-voltage electrode 412 and each of those members 416 and
421 are sufficiently long, the reference potential is spread over a
wide range within the space between the high-voltage electrode 412
and each of those members 416 and 421, preventing the development
of the creeping discharge. However, in the previously described
conventional Ar-BID, since the ground electrodes 413 and 414 are
not sufficiently long, the creeping discharge originating from the
high-voltage electrode 412 can develop beyond the areas where the
ground electrodes 413 and 414 are located, to eventually reach the
tube-line tip member 416 or the charge-collecting section, causing
the aforementioned expansion of the plasma generation area. Based
on such an inference, the present inventors have compared the
initiation distance for the creeping discharge in the Ar gas and
the same distance in the He gas. The result confirmed that the
creeping discharge initiation distance in the Ar gas was longer
(i.e. the creeping discharge could occur at a longer distance
between the high-voltage electrode 412 and each of the members 416
and 421).
[0021] Given this result, the present inventors measured the SN
ratio in each case where only one of the ground electrodes 413 and
414 above and below the high-voltage electrode 412 was made longer
than the conventional ones. The result demonstrated that the SN
ratio particularly improved when the downstream-side ground
electrode 414 was made longer. A likely reason for this improvement
is that, if the creeping discharge occurs in the downstream area of
the flow of the plasma generation gas, i.e. in the area near the
charge-collecting section, and causes the plasma generation area to
expand into the downstream area, the collecting electrode provided
for detecting the ion current in the charge-collecting section
suffers from the mixing of electromagnetic noise due to the high
voltage or an incidence of charged particles from the plasma.
[0022] These facts suggest that, in order to prevent the creeping
discharge in sur Ar-BID, it is beneficial to make the ground
electrodes, and particularly the one located on the downstream side
of the high-voltage electrode, longer than the initiation distance
for the creeping discharge in the Ar-BID. However, as shown in FIG.
6, increasing the length of a ground electrode requires increasing
the length of the dielectric tube 511 to which the ground electrode
514 is attached, which consequently increases the entire size of
the detector. In BIDs, since the charge-collecting section is
normally heated to 200.degree. C. or higher temperatures to
maintain the sample gas in the gasified state, increasing the
detector size leads to an increase in the degree of non-uniformity
in the temperature within the dielectric tube, which causes a
fluctuation of the output signal.
[0023] Thus, a dielectric harrier discharge ionization detector
according to one aspect of the resent invention developed for
solving the previously described problem is a dielectric barrier
discharge ionization detector for ionizing and detecting a sample
component in a sample gas by using plasma induced by an electric
discharge within a gas passage through which a plasma generation
gas containing argon is passed, the detector including:
[0024] a) a dielectric tube made of a dielectric material and
containing an upstream section of the gas passage;
[0025] b) a high-voltage electrode attached to the outer wall of
the dielectric tube;
[0026] c) a ground electrode electrically connected to a ground and
arranged so as to face the gas passage, the ground electrode having
a surface which laces the gas passage and is covered with a
dielectric body, with at least a portion of the surface being
located downstream of the high-voltage electrode in the flow
direction of the plasma generation gas;
[0027] d) an AC power source connected to the high-voltage
electrode, for applying an AC voltage between the high-voltage
electrode and the ground electrode so as to induce a dielectric
barrier discharge within the gas passage and thereby generate
plasma; and
[0028] e) a charge-collecting section located downstream of the
ground electrode, forming a downstream section of the gas passage,
including a sample-gas introducer for introducing a sample gas into
the downstream section, and a collecting electrode for collecting
ions generated from a sample component in the sample gas by light
emitted from the plasma,
[0029] where:
[0030] a semiconductor film is formed on the inner wall of the
dielectric tube over an area where the high-voltage electrode is
attached and/or an area located downstream of the aforementioned
area and upstream of the charge-collecting section; and
[0031] the length of the ground electrode in the area located
downstream of the high-voltage electrode is longer than the
initiation distance for a creeping discharge between the
high-voltage electrode and the charge-collecting section.
[0032] The "initiation distance for a creeping discharge between
the high-voltage electrode and the charge-collecting section" means
the largest length of the ground electrode which allows the
creeping discharge to occur between the high-voltage electrode and
the ground electrode under the condition that the ground electrode
covered with a dielectric body is located between the high-voltage
electrode and the charge-collecting section (the same applies
below). For example, in the Ar-BID having the discharging section
410 as shown in FIG. 4 in which the ground electrode 414 is
arranged between the high-voltage electrode 412 and the
charge-collecting section located downstream of the discharging
section 410, if the length of the ground electrode 414 is gradually
decreased while the high AC excitation voltage power source 415 is
energized, a creeping discharge begins at a certain length, causing
a sudden increase in the current value as measured between the
high-voltage electrode 412 and the charge-collecting section (e.g.
the connection member 421). The length of the ground electrode 414
at which such a sudden increase in the current value occurs is the
initiation distance for the creeping discharge.
[0033] During the development of the creeping discharge on the
surface of a dielectric body, the ionization actively occurs at its
front end, forming a highly conductive path behind. However, if a
semiconductor film is present on the surface of the dielectric
body, the electric charges resulting from the ionization are
rapidly diffused through the semiconductor film. Therefore, the
aforementioned highly conductive path will not be formed and the
creeping discharge will not easily develop. Accordingly, the
development of the creeping discharge from the high-voltage
electrode toward the charge-collecting section can be impeded by
forming a semiconductor film on the inner wall of the dielectric
tube over the area where the high-voltage electrode is attached
and/or the area located downstream of the aforementioned area and
upstream al the charge-collecting section as in the present
invention. As a result, the creeping discharge initiation distance
within the area between the high-voltage electrode and the
charge-collecting section becomes shorter than in the conventional
case. Therefore, making the ground electrode longer than the
creeping discharge initiation distance does not cause a significant
increase in the detector size. That is to say, in the dielectric
barrier discharge ionization detector according, to the present
invention, it is possible to prevent an occurrence of the creeping
discharge while minimizing the increase the detector size.
[0034] Although there is no specific limitation on the kind of
semiconductor to form the semiconductor film, it is preferable to
use a semiconductor which allows for easy formation of the film on
the inner wall of the dielectric tube and which also has a low
level of reactivity that makes the film hard to be sputtered.
Examples of semiconductors having such characteristics include
diamond-like carbon and titanium oxide.
[0035] The initiation distance for a creeping discharge depends on
the kind of semiconductor forming the semiconductor film, the
thickness, area and other properties of the semiconductor film as
well as such parameters as the frequency and amplitude of the
low-frequency AC voltage, waveform of the power source, property of
gas (gas purity), and material of the dielectric body. Therefore,
"the length of the ground electrode in the area located downstream
of the high-voltage electrode" in the present invention should be
determined according to those parameters to be applied when the
Ar-BID is in use.
[0036] The dielectric barrier discharge ionization detector
according to the present invention is not limited to a
configuration as shown in FIG. 4 in which a high-voltage electrode
412 and two ground electrodes 413 and 414 are circumferentially
formed on the outer circumferential surface of a cylindrical
dielectric tube 411; it can be applied in various structures of the
dielectric barrier discharge ionization detectors. For example, the
present invention can also be applied in a dielectric barrier
discharge ionization detector disclosed in Patent Literature 3 (a
schematic configuration of which is shown in FIG. 7), which
includes a high-voltage electrode 612 circumferentially formed on
the outer circumferential surface of an external dielectric tube
611 and an electrode structure 634 inserted into the external
dielectric tube 611, the electrode structure 634 including an
electrically grounded metallic tube 632 (which corresponds to the
ground electrode in the present invention) covered with an internal
dielectric tube 631 (this example will be detailed later).
[0037] When the present invention is applied in the dielectric
harrier discharge ionization detector as shown in FIG. 4, the
cylindrical dielectric tube 411 corresponds to both the "dielectric
tube" and the "dielectric body" covering the ground electrode in
the present invention. In other words, the dielectric tube and the
dielectric body are formed as a single component. When the present
invention is applied in the dielectric harrier discharge ionization
detector as shown in FIG. 7, the external dielectric tube 611
corresponds to the "dielectric tube" in the present invention,
while the internal dielectric tube 631 corresponds to the
"dielectric body" covering the ground electrode in the present
invention. In other words, the dielectric tube and the dielectric
body are formed as separate components.
[0038] A dielectric barrier discharge ionization detector according
to another aspect of the present invention is a dielectric barrier
discharge ionization detector for ionizing and detecting a sample
component in a sample gas by using plasma induced by an electric
discharge within a gas passage through which a plasma generation
gas containing argon is passed, the detector including:
[0039] a) a dielectric tube made of a dielectric material and
containing an upstream section of the gas passage;
[0040] b) an electrically grounded tube-line tip member made of
metal, for introducing the plasma generation gas into the
dielectric tube;
[0041] c) a high-voltage electrode attached to the outer wall of
the dielectric tube;
[0042] d) an electrically grounded upstream-side ground electrode
attached to the outer wall of the dielectric tube and located
upstream of the high-voltage electrode as well as downstream of the
tube-line tip member in the flow direction of the plasma generation
gas;
[0043] e) an electrically grounded downstream-side ground electrode
attached to the outer wall of the dielectric tube and located
downstream of the high-voltage electrode in the flow direction of
the plasma generation gas;
[0044] f) an AC power source connected to the high-voltage
electrode, for applying an AC voltage between the high-voltage
electrode and the upstream-side ground electrode as well as between
the high-voltage electrode and the downstream-side ground electrode
so as to induce a dielectric barrier discharge within the gas
passage and thereby generate plasma; and
[0045] g) a charge-collecting section located downstream of the
downstream-side ground electrode, for ling a downstream section of
the gas passage, including a sample-gas introducer for introducing
a sample gas into the downstream section, and a collecting
electrode for collecting ions generated from a sample component in
the sample gas by light emitted from the plasma,
[0046] where:
[0047] a semiconductor film is formed on the inner wall of the
dielectric tube over an area where the high-voltage electrode is
attached and/or an area located downstream of the aforementioned
area and upstream of the charge-collecting section; and
[0048] the length of the downstream-side ground electrode in the
flow direction is longer than the initiation distance for a
creeping discharge between the high-voltage electrode and the
charge-collecting section.
[0049] The above-described configuration is the case where the
present invention is applied in a dielectric barrier discharge
ionization detector configured as shown in FIG. 4 in which a
high-voltage electrode 412 and two ground electrodes 413 and 414
are circumferentially formed on the outer circumferential surface
of a cylindrical dielectric tube 411.
[0050] As noted earlier, in the dielectric barrier discharge
ionization detector, the tube-line tip member 416 provided at the
upper end of the discharging section 410 is also made of metal and
electrically grounded. Therefore, the creeping discharge
originating from the high-voltage electrode 412 can develop not
only on the downstream side but also on the upstream side of the
same electrode.
[0051] Accordingly, the dielectric barrier discharge ionization
detector according to the previously described aspect of the
present invention may be modified into a dielectric harrier
discharge ionization detector for ionizing and detecting a sample
component in a sample gas by using plasma induced by an electric
discharge within a gas passage through which a plasma generation
gas containing argon is passed, the detector including:
[0052] a) a dielectric tube made of a dielectric material and
containing an upstream section of the gas passage;
[0053] b) an electrically grounded tube-line tip member made of
metal, for introducing the plasma generation gas into the
dielectric tube;
[0054] c) a high-voltage electrode attached to the outer wall of
the dielectric tube;
[0055] d) an electrically grounded upstream-side ground electrode
attached to the outer wall of the dielectric tube and located
upstream of the high-voltage electrode as well as downstream of the
tube-line tip member in the flow direction of the plasma generation
gas;
[0056] e) an electrically grounded downstream-side ground electrode
attached to the outer wall of the dielectric tube and located
downstream of the high-voltage electrode in the flow direction of
the plasma generation gas;
[0057] f) an AC power source connected to the high-voltage
electrode, for applying an AC voltage between the high-voltage
electrode and the upstream-side ground electrode as well as between
the high-voltage electrode and the downstream-side ground electrode
so as to induce a dielectric barrier discharge within the gas
passage and thereby generate plasma; and
[0058] g) a charge-collecting section located downstream of the
downstream-side ground electrode, forming a downstream section of
the gas passage, including a sample-gas introducer for introducing
a sample gas into the downstream section, and a collecting
electrode fir collecting ions generated from a sample component in
the sample gas by light emitted from the plasma,
[0059] where:
[0060] a semiconductor film is formed on the inner wall of the
dielectric tube over an area where the high-voltage electrode is
attached and/or an area located upstream of the aforementioned area
as well as downstream of the tube-line tip member; and
[0061] the length of the upstream-side ground electrode in the flow
direction is longer than the initiation distance tzar a creeping
discharge between the high-voltage electrode and the tube-line tip
member.
[0062] For the purpose of improving the SN ratio, it is preferable
make both of the upstream-side and downstream-side ground
electrodes longer than the initiation distance for the creeping
distance on the upstream and downstream sides of the high-voltage
electrode, respectively. However, it causes some problems, such as
an increase in the entire length of the dielectric tube, as well as
the necessity to supply a considerably high voltage from the AC
power source since a configuration for completely preventing the
creeping discharge also impedes the intended electric discharge
between the high-voltage electrode and each of the upper and lower
ground electrodes. Accordingly, the configuration in which only one
of the ground electrodes is made longer also has practical
merits.
Advantageous Effects of the Invention
[0063] As described to this point, the dielectric barrier discharge
ionization detector (Ar-BID) according to the present invention
configured in the previously described manner can prevent an
occurrence of the creeping discharge, whereby the single-side
barrier discharge mentioned earlier is prevented and the SN ratio
is improved.
BRIEF DESCRIPTION OF DRAWINGS
[0064] FIG. 1 is a schematic configuration diagram of an Ar-BID
according, to the first embodiment of the present invention.
[0065] FIG. 2 shows the electrode arrangement in a test example and
a comparative example.
[0066] FIG. 3 is a schematic configuration diagram of an Ar-BID
according to the second embodiment of the present invention.
[0067] FIG. 4 is a schematic configuration diagram of the
discharging section and surrounding area in a conventional BID.
[0068] FIG. 5 is a graph showing the relationship between the
discharge initiation voltage for spark discharge in Ar and He at
atmospheric pressure and the inter-electrode distance.
[0069] FIG. 6 is a schematic configuration diagram show no a case
in which the downstream-side ground electrode of a conventional BID
is made longer.
[0070] FIG. 7 is a schematic configuration diagram showing still
another configuration example of conventional BIDs.
DESCRIPTION OF EMBODIMENTS
[0071] Modes for carrying out the present invention are hereinafter
described using embodiments.
First Embodiment
[0072] FIG. 1 is a schematic configuration diagram of an Ar-BID
according to the first embodiment of the present invention.
[0073] The Ar-BID of the present embodiment includes a cylindrical
dielectric tube 111 made of a dielectric material (e.g. quartz
glass) through which a plasma generation gas is passed. In the
following description, for convenience of explanation, the vertical
direction is defined in such a manner that the upstream side in the
flow direction of the gas (indicated by the downward arrows in FIG.
1) in the cylindrical dielectric tube 111 is called the "upper"
side, and the downstream side is called the "lower" side. However,
this definition does not limit the direction in which the Ar-BID
should be used.
[0074] On the inner wall surface of the cylindrical dielectric tube
111, a semiconductor film 117 is formed over the entire length of
the tube (as will be detailed later). On the outer wall surface al
the cylindrical dielectric tube 111, three ring-shaped electrodes
made of an electric conductor (e.g. stainless steel or copper) are
circumferentially formed at predetermined intervals of space along
the flow direction of the gas.
[0075] Among the three electrodes, the central electrode 112 has a
high AC excitation voltage power source 115 connected, while the
two electrodes 113 and 114 located above and below the electrode
112 are both grounded. Hereinafter, the electrodes 112, 113 and 114
are called the "high-voltage electrode", "upstream-side ground
electrode" and "downstream-side ground electrode", respectively,
and these electrodes are collectively called the "plasma generation
electrodes". The high AC excitation voltage power source 115
generates a high AC voltage at a frequency within a range of 1
kHz-100 kHz, more preferably, approximately 5 kHz-30 kHz (low
frequency), with an amplitude of approximately 5 kV-10 kV. The AC
voltage may have any waveform, such as a sinusoidal, rectangular,
triangular or sawtooth wave.
[0076] In the Ar-BID of the present embodiment, the area above the
lower end of the downstream-side ground electrode 114 in FIG. 1 is
the discharging section 110, and the area below the lower end of
the downstream-side ground electrode 114 is the charge-collecting
section 120.
[0077] The cylindrical dielectric tube 111 has a tube-line tip
member 116 at its upper end, to which a gas supply tube 116a is
connected. Through this gas supply tube 116a, a plasma generation
gas (Ar gas, or He gas with a trace amount of Ar gas added)
doubling as a dilution gas is supplied into the cylindrical
dielectric tube 11 I. Since the wall surface of the cylindrical
dielectric tube 111 is present between the plasma generation gas
and each of the plasma generation electrodes 112, 113 and 114, the
wall surface itself functions as the dielectric coating layer which
covers the surfaces of the plasma generation electrodes 112, 113
and 114, enabling dielectric harrier discharge to occur, as will be
described later.
[0078] On the downstream side of the cylindrical dielectric tube
111, a connection member 121, bias electrode 122 and collecting
electrode 123, all of which are cylindrical bodies having the same
inner diameter, are arranged along the flow direction of the gas,
with insulators 125a and 125b made of alumina, PTFE
(polytetrafluoroethylene) resin or similar material inserted in
between. On the downstream side of the collecting electrode 123, a
tube-line end member 124 in the form of a cylindrical body with a
closed bottom is attached via an insulator 125c. The inner space
formed by the connection member 121, bias electrode 122, collecting
electrode 123, tube-line end member 124 and insulators 125a, 125b
and 125c communicates with the inner space of the cylindrical
dielectric tube 111.
[0079] A bypass exhaust tube 121a for exhausting a portion of the
plasma generation gas to the outside is connected to the
circumferential surface of the connection member 121. A sample
exhaust tube 124a is connected to the circumferential surface of
the tube-line end member 124. A thin sample introduction tube 126
is inserted through the bottom of the tube-line end member 124.
Through this sample introduction tube 126, a sample gas is supplied
into the charge-collecting section 120. The charge-collecting
section 120 is heated to a maximum temperature of approximately
450.degree. C. by an external heater (not shown) in order to
maintain the sample gas in the gasified state.
[0080] The connection member 121 is grounded and functions as a
recoil electrode for preventing charged particles in the plasma
carried by the gas stream from reaching the collecting electrode
123. The bias electrode 122 is connected to a bias DC power source
127. The collecting electrode 123 is connected to a current
amplifier 128.
[0081] The operation for detecting a sample component contained in
a sample gas in the present Ar-BID is hereinafter schematically
described. As indicated by the rightward arrow in FIG. 1, a plasma
generation gas doubling as a dilution gas is supplied through the
gas supply tube 116a into the cylindrical dielectric tube 111.
Since the BID according to the present embodiment is an Ar-BID,
either an Ar gas or a He gas containing a trace amount of Ar gas is
used as the plasma generation gas. The plasma generation gas flows
downward through the cylindrical dielectric tube 111, a portion of
which is exhausted through the bypass exhaust tube 121a to the
outside, while the remaining portion serving as the dilution gas
flows downward through the charge-collecting section 120, to be
exhausted through the sample exhaust tube 124a to the outside.
Meanwhile, the sample as containing the sample component is
supplied through the sample introduction tube 126 and ejected from
the sample-gas ejection port at the end of the same tube into the
charge-collecting section 120. Although the direction in which the
sample gas is ejected from the sample-gas ejection port is opposite
to the flow direction of the dilution gas, the sample gas is
immediately pushed backward, being merged with the dilution gas and
flowing downward, as indicated by the arrows in FIG. 1.
[0082] As noted earlier, while the plasma generation gas is flowing
through the cylindrical dielectric tube 111, the high AC excitation
voltage power source 115 applies a high AC voltage between the
high-voltage electrode 112 and the upstream-side ground electrode
113 as well as between the high-voltage electrode 112 and the
downstream-side ground electrode 114. As a result, a dielectric
barrier discharge occurs within the cylindrical dielectric tube
111, whereby the plasma generation gas is ionized and a cloud of
plasma (atmospheric pressure non-equilibrium plasma) is generated.
The excitation light emitted from the atmospheric-pressure
non-equilibrium plasma travels through the discharging section 110
and the charge-collecting section 120 to the region where the
sample gas is present, and ionizes the sample component in the
sample gas. The thereby generated ions move toward the collecting
electrode 123 due to the effect of the electric field created by
the DC voltage applied to the bias electrode 122. Upon reaching the
collecting electrode 123, the ions give electrons to or receive
electrons from the same electrode. Consequently, an ion current
corresponding to the amount of ions generated from the sample
component, i.e. an ion current corresponding to the amount of
sample component, is fed to the current amplifier 128. The current
amplifier 128 amplifies this current and produces a detection
signal. In this manner, the Ar-BID according to the present
embodiment produces a detection signal corresponding to the amount
(concentration) of the sample component contained in the sample gas
introduced through the sample introduction tube 126.
[0083] The basic components of the BID in the present embodiment
are the same as those of commonly used BIDs. The previously
described basic operation for detection is also similar to that of
commonly used BIDs. The structural characteristics of the Ar-BID
according to the present embodiment exist in that the inner wall
surface of the cylindrical dielectric tube 111 is covered with a
semiconductor film 117, and that the upstream-side ground electrode
113 and the downstream-side ground electrode 114 are respectively
made longer than the creeping discharge initiation distances
between the high-voltage electrode 112 and the tube-line tip member
116 as well as between the high-voltage electrode 112 and the
charge-collecting section 120 (more specifically, the connection
member 121).
[0084] When the semiconductor film 117 is formed on the inner wall
surface of the cylindrical dielectric tube 111 in the previously
described manner, the creeping discharge initiation distances
between the high-voltage electrode 112 and the tribe-line tip
member 116 as well as between the high-voltage electrode 112 and
the connection member 121 become shorter than when no semiconductor
film 117 is formed. As a result, the length of the upstream-side
ground electrode 113 and that of the downstream-side ground
electrode 114 necessary for preventing the creeping discharge also
become shorter than when no semiconductor film 117 is formed (e.g.
than the electrode 514 in FIG. 6). Accordingly, in the Ar-BID
according to the present embodiment, it is possible to prevent an
occurrence of the creeping discharge and thereby improve the SN
ratio while minimizing the increase in the detector size.
[0085] Although the semiconductor film 117 ay be any kind of
material, it is preferable to use a semiconductor which has a low
level of reactivity that makes the film resistant to sputtering and
which also allows for easy formation of the film on the inner wall
of the cylindrical dielectric tube 111. For example, a diamond-like
carbon (DLC) film which cast be formed by plasma chemical vapor
deposition (CVD) or a titanium oxide (TiO.sub.2) film which can be
formed by a sol-gel process can be suitably used as the
semiconductor film.
[0086] In the present embodiment, the length of the downstream-side
ground electrode 114 is adjusted according to the kind and
thickness of the semiconductor film as well as such parameters as
the frequency and amplitude of the low-frequency AC voltage,
waveform of the power source, and property of gas so that no
creeping discharge will occur between the high-voltage electrode
112 and the charge-collecting section 120. The length of the
upstream-side ground electrode 113 is also adjusted according to
the kind and thickness of the semiconductor film as well as the
aforementioned parameters so that no creeping discharge will
between the high-voltage electrode 112 and the tube-line tip member
116.
[0087] In the previously described example, the semiconductor film
117 is formed over the entire length of the cylindrical dielectric
tube 111. However, the minimum requirement is to form the
semiconductor film 117 on at least one of the following portions of
the cylindrical dielectric tube 111: the inner circumferential
surface of the area which covers the high-voltage electrode 112
(this area is hereinafter called the "high-voltage electrode
coverage area"); the inner circumferential surface of the area
located downstream of the high-voltage electrode coverage area as
well as upstream of the charge-collecting section 120 (this area is
hereinafter called the "downstream-side area"); and the inner
circumferential surface of the area located upstream of the
high-voltage electrode coverage area as well as downstream of the
tube-line tip member 116 (this area is hereinafter called the
"upstream-side area"). Providing, the semiconductor film on the
high-voltage electrode coverage area and/or the downstream-side
area shortens the creeping discharge initiation distance between
the high-voltage electrode 112 and the charge-collecting section
120. Providing the semiconductor film on the high-voltage electrode
coverage area and/or the upstream-side area shortens the creeping
discharge initiation distance between the high-voltage electrode
112 and the tube-line tip member 116.
[0088] In the previously described example, both the upstream-side
ground electrode 113 and the downstream-side ground electrode 114
are made longer than the respective creeping discharge initiation
distances. It is also possible to make only one of them, and
particularly, only the downstream-side ground electrode 114, longer
than the creeping discharge initiation distance (between the
high-voltage electrode 112 and the charge-collecting section 120).
In the case of making only the downstream-side ground electrode 114
longer than the creeping discharge initiation distance, the
semiconductor film is formed on the inner circumferential surface
of at least either be high-voltage electrode coverage area or the
downstream-side area. In the case of making only the upstream-side
ground electrode 113 longer than the creeping discharge initiation
distance, the semiconductor film is formed on the inner
circumferential surface of at least either the high-voltage
electrode coverage area or the upstream-side area.
TEST EXAMPLE
[0089] Hereinafter described is a test conducted for confirming the
effect of the Ar-BID according to the first embodiment. The test
was performed using an Ar-BID in which a semiconductor film made of
DLC was added to a portion of the inner circumferential surface of
the cylindrical dielectric tube (this BID is hereinafter called the
"test example") as well as an Ar-BID including the cylindrical
dielectric tube with no semiconductor film added (this BID is
hereinafter called the "comparative example"). FIG. 2 shows the
electrode arrangement in the discharging section of the Ar-BIDS in
the test example and the comparative example. It should be noted
that the semiconductor film 217 was formed only in the test
example, at the shown location. In both of the test and comparative
examples, the cylindrical dielectric tube 211 was a quartz tube
measuring 4 mm in outer diameter, 2 mm in inner diameter and 92 mm
in length. Strips of copper toil were wound on the outer
circumferential surface of the cylindrical dielectric tube 211 to
thrill the high-voltage electrode 212, upstream-side ground
electrode 213 and downstream-side ground electrode 214. The
electrode arrangement in FIG. 2 was determined so that a creeping
discharge would occur on the upstream side of the high-voltage
electrode 212 while no creeping discharge would occur on the
downstream side when the measurement was performed using the
cylindrical dielectric tube 211 with no semiconductor film under
measurement conditions which will be described later. In other
words, the upstream-side ground electrode 213 was shorter than the
initiation distance for the creeping discharge between the
high-voltage electrode 212 and the tube-line tip member 216 under
the aforementioned conditions in the Ar-BID of the comparative
example, while the downstream-side ground electrode 214 was longer
than the initiation distance for the creeping discharge between the
high-voltage electrode 212 and the connection member 221 under the
same conditions in the Ar-BID of the comparative example.
[0090] Using each of those Ar-BIDs as the detector for GC, the
sensitivity for a solution of a standard sample (dodecane) was
measured, with Ar gas (with a degree of purity of 99.9999% or
higher) continuously introduced into the cylindrical dielectric
tube 211, and the high AC excitation voltage power source 215
energized to apply an AC high voltage having a sinusoidal current
waveform at a frequency of approximately 40 kHz with a voltage
amplitude of approximately 5 kVp-p. The detection limit was also
calculated for each case from the measured noise value. Table 1
below shows the measured results and the calculated results based
on the measured results.
TABLE-US-00001 TABLE 1 Sensitivity Noise Detection Limit (C/g) (fA)
(pg/sec) Test Example 0.78 90 0.23 Comparative Example 1.0 135
0.27
[0091] As shown in Table 1, the sensitivity in the test example was
lower than in the comparative example. This seems to be due to the
fact that the shortened initiation distance for the creeping
discharge as compared to the comparative example suppresses not
only the creeping discharge developing from the high-voltage
electrode 212 into the downstream area but also the creeping
discharge developing into the upstream area, causing the discharge
area on the upstream side of the high-voltage electrode 212 to
shrink and the amount of discharge light to decrease. However, in
the test example, not only the sensitivity but also the noise value
was decreased, with the result that the detection limit was higher
than in the comparative example. The SN ratio was also higher than
in the comparative example. The most likely explanation for this
improvement in the SN ratio is that the shrinkage of the discharge
area on the upstream side of the high-voltage electrode 212 enabled
the electric discharge on the upstream side, which occurs as the
single-side barrier discharge in the comparative example, to be a
double-side barrier discharge in the test example.
[0092] As just described, in the test example, the SN ratio could
be improved even on the upstream area where the ground electrode
213 had a relatively short length. Therefore, in the Ar-BID of the
test example, it should be possible to make the downstream-side
ground electrode 214 as short as the upstream-side ground electrode
213 and yet prevent the creeping discharge from developing from the
high-voltage electrode 212 into the downstream area, by extending
the area of the semiconductor film 217 (DLC film) on the inner wall
of the cylindrical dielectric tube 211 into the area located
downstream of the high-voltage electrode 212.
[0093] As noted earlier, the initiation distance for a creeping
discharge depends on such parameters as the kind, thickness and
area of the semiconductor film as well as the frequency and
amplitude of the low-frequency AC voltage, waveform of the power
source, property of gas, and material of the dielectric body.
Accordingly, the lengths of the ground electrodes in the Ar-BID)
according to the present embodiment are not limited to the values
shown in FIG. 2, but should be appropriately determined according
to the configuration and use conditions of the Ar-BID. For example,
under the condition that the various aforementioned parameters are
fixed, the length of the downstream-side or upstream-side ground
electrode may be variously changed to locate a point at which a
sudden change occurs in a specific quantity, such as the size of
the plasma generation area, amount of the current flowing between
the high-voltage electrode (112 in FIG. 1) and the connection
member (121 in FIG. 1), amount of the current flowing between the
high-voltage electrode and the tube-line tip member (116 in FIG.
1), or SN ratio during a sample measurement. The length of the
ground electrode at the located point can be considered to
correspond to the initiation distance for the creeping discharge.
Therefore, by making the actual length of the downstream-side or
upstream-side ground electrode larger than the length corresponding
to the aforementioned point, the creeping discharge can be
suppressed and a high SN ratio can be achieved. Furthermore, if the
length of the ground electrode at which the creeping discharge
ceases is previously investigated with various values of the
aforementioned parameters, it becomes possible to estimate the
length of the ground electrode with which a high SN ratio can be
achieved under a given condition.
Second Embodiment
[0094] The second embodiment of the Ar-BID according to the present
invention is hereinafter described with reference to FIG. 3. FIG. 3
is a schematic configuration diagram of the Ar-BID according to the
present embodiment.
[0095] The Ar-BID of the present embodiment includes an external
dielectric tube 311 made of a dielectric material, such as quartz.
For example, a quartz tube measuring 7 mm in outer diameter and 5
mm in inner diameter can be used as the external dielectric tube
311. A semiconductor film 317 is formed over the entire length of
the inner wall surface of the external dielectric tube 311 (as will
be detailed, later). A ring-shaped electrode 312 made of metal
(e.g. stainless steel or copper) is circumferentially formed on the
outer circumferential surface of the external dielectric tube
311.
[0096] At the upper end of the external dielectric tube 311, a
tube-line tip member 316 having a cylindrical shape with a closed
top and an open bottom is attached. A gas supply tube 316a is
connected to the circumferential surface of the tube-line tip
member 316. The tube-line tip member 316 and the gas supply tube
316a are made of metal, such as stainless steel.
[0097] Inside the external dielectric tube 311, an internal
dielectric tube 331 made of a dielectric material (e.g. quartz) is
arranged, and a metallic tube 332 made of metal (e.g. stainless
steel) is inserted into this internal dielectric tube 331.
Furthermore, an insulating tube 333 made of alumina or the like is
inserted into the metallic tube 332, and a metallic wire 322 made
of metal (e.g. stainless steel) is inserted into the insulating
tube 333. The internal dielectric tube 331, metallic tube 332,
insulating tube and metallic wire 322 have their respective lengths
sequentially increased in the mentioned order, with the upper and
lower ends of the metallic tube 332 protruding from the upper and
lower ends of the internal dielectric tube 331, and the upper and
lower ends of the insulating tube 333 protruding from the upper and
lower ends of the metallic tube 332. The upper and lower ends of
the metallic wire 322 also protrude from the upper and lower ends
of the insulating tube 333. The structure composed a the internal
dielectric tube 331, metallic tube 332, insulating tube 333 and
metallic wire 322 is hereinafter called the "electrode structure
334".
[0098] The tube-line tip member 316 has a through hole formed in
its upper portion. The upper end of the metallic tube 332 is fixed
in this through hole by welding or soldering. The insulating tube
333 and the metallic wire 322 are extracted to the outside from the
through hole in the upper portion of the tube-line tip member 316,
and sealed and fixed on the upper surface of the tube-line tip
member 316 with a gas-tight adhesive 316b.
[0099] The tube-line up member 316 is electrically grounded through
an electric line (or gas supply tube 316a), whereby the metallic
tube 332 is also electrically grounded via the tube-line tip member
316. On the other hand, the ring-shaped electrode 312 has a high AC
excitation voltage power source 315 connected to it. That is to
say, in the Ar-BID of the present embodiment, the ring-shaped
electrode 312 corresponds to the high-voltage electrode in the
present invention, the area of the metallic tube 332 covered with
the internal dielectric tube 331 (this area is hereinafter called
the "dielectric coverage area") corresponds to the ground electrode
in the present invention, and the ring-shaped electrode
(high-voltage electrode) 312 and the dielectric coverage area of
the metallic tube 332 (ground electrode) function as the plasma
generation electrodes. The inner circumferential surface of the
ring-shaped electrode 312 faces a portion of the outer
circumferential surface of the metallic tube 332 across the wall
surfaces of the external dielectric tube 311 and the internal
dielectric tube 331. Accordingly, those dielectric wall surfaces
themselves function as dielectric coating layers which cover the
surfaces of the plasma generation electrodes (i.e. ring-shaped
electrode 312 and metallic tube 332), enabling a dielectric barrier
discharge to occur.
[0100] In the present embodiment, the area above the lower end of
the internal dielectric tube 331 in FIG. 3 corresponds to the
discharging section 310, and the area below the lower end of the
internal dielectric tube 331 corresponds to the charge-collecting
section 320.
[0101] The lower end of the external dielectric tube 311 is
inserted into the cylindrical connection member 321. A bypass
exhaust tube 321a made of metal (e.g. stainless steel) is provided
on the circumferential surface of the connection member 321.
[0102] Below the connection member 321, there are a cylindrical
insulating member 325a, flanged metallic tube 323, cylindrical
insulating member 325b, and tube-line end member 324 arranged in
the mentioned order. The flanged metallic tube 323 has a
cylindrical portion 323a and a flange portion 323b which is formed
at the lower end of the cylindrical portion 123a and extends
outward in the radial direction of the cylindrical portion 323a.
The cylindrical portion 323a has an outer diameter smaller than the
inner diameter of the external dielectric tube 311 and is inserted
in the external dielectric tube 311 from below. The flange portion
323b, which has approximately the same outer diameter as those of
the connection member 321, insulating members 325a, 325b and
tube-line end member 324, is held between the lower end of the
connection member 321 and the upper end of the tube-line end member
324 via the insulating members 325a and 325b. The connection member
321, tube-line end member 324 and flanged metallic tube 323 are all
made of metal (e.g. stainless steel). The connection member 321,
insulating member 325a, flanged metallic tube 323, insulating
member 325b, and tube-line end member 324 are each adhered to the
neighboring members with a heat-resistant ceramic adhesive.
[0103] The tube-line end member 324 is a cylindrical member having
an open top and a closed bottom, with a sample exhaust tube 324a
made of metal (e.g. stainless steel) connected to its
circumferential surface. A through hole is formed in the bottom
surface of the tube line end member 324, and a sample introduction
tube 326 connected to the exit end of a GC column (or similar
element) is inserted into the through hole. The sample introduction
tube 326 is pulled into the cylindrical portion 323a of the flanged
metallic tube 323. The upper end (i.e. sample-gas exit port) of the
sample introduction tube 326 is located at a vertical position
between the upper and lower ends of the cylindrical portion
323a.
[0104] As described earlier, a portion which is not covered with
the insulating tube 333 ("exposed portion") is provided at the
lower end of the metallic wire 322 in the electrode structure 334.
The exposed portion is inserted into the cylindrical portion 323a
of the flanged metallic tube 323 from above and is located near the
upper end of the cylindrical portion 323a. As a result, the exposed
portion of the metallic wire 322 is located directly above the
sample-gas exit port. Furthermore, the metallic wire 322 is
extracted from the tube-line tip member 316 to the outside and
connected to a bias DC power source 327. The flanged metallic tube
323 is connected to a current amplifier 328. That is to say, in the
Ar-BID of the present embodiment, the exposed portion at the lower
end of the metallic wire 322 functions as the bias electrode, while
the cylindrical portion 323a of the flanged metallic tube 323
functions as the ion-collecting electrode. Accordingly, the space
between the inner wall of the cylindrical portion 323a and the
exposed portion of the metallic wire 322 is the effective
ion-collecting area.
[0105] As noted earlier, the metallic tube 332 included in the
electrode structure 334 is grounded via the tube-line tip member
316, and a portion which is not covered with the internal
dielectric tube 331 ("exposed portion") is provided at the lower
end of the metallic tube 332. This exposed portion is located
directly above the flanged metallic tube 323 and functions as a
recoil electrode for preventing charged particles in the plasma
from reaching the ion-collecting electrode (i.e. the cylindrical
portion 323a).
[0106] A detecting operation by the present Ar-BID is hereinafter
described. As indicated by the rightward arrow in FIG. 3, a plasma
generation gas (Ar gas, or He gas containing a trace amount of Ar
gas) doubling as a dilution gas is supplied through the gas supply
tube 316a into the tube-line tip member 316.
[0107] The plasma generation gas doubling as the dilution gas flows
downward through the space between the inner wall of the external
dielectric tube 311 and the outer wall of the internal dielectric
tube 331. At the upper end of the cylindrical portion 323a of the
flanged metallic tube 323, a portion of the gas is made to branch
off The branch portion of the plasma generation gas flows downward
through the space between the inner wall of the external dielectric
tube 311 and the outer wall of the cylindrical portion 323a. At the
lower end of the external dielectric tube 311, the flow turns
outward, and then upward. After flowing upward through the space
between the outer wall of the external dielectric tube 311 and the
inner wall of the connection member 321, the gas is exhausted
through the bypass exhaust tube 321a to the outside. Meanwhile, the
remaining portion of the plasma generation gas flows into the space
surrounded by the inner wall of the cylindrical portion 323a, as
the dilution gas to be mixed with the sample gas.
[0108] While the plasma generation gas is flowing through the space
between the inner wall of the external dielectric tube 311 and the
outer wall of the internal dielectric tube 331 in the previously
described manner, the high AC excitation voltage power source 315
is energized. The high AC excitation voltage power source 315
applies a low-frequency high AC voltage between the plasma
generation electrodes, i.e. the ring-shaped electrode (high-voltage
electrode) 312 and the dielectric coverage area (ground electrode)
of the metallic tube 332. Consequently, an electric discharge
occurs within the area sandwiched between the ring-shaped electrode
312 and the dielectric coverage area of the metallic tube 332. This
electric discharge is induced through the dielectric coating layers
(external dielectric tube 311 and internal dielectric tube 331),
and therefore, is a dielectric barrier discharge. By this
dielectric barrier discharge, the plasma generation gas flowing
through the space between the inner wall of the external dielectric
tube 311 and the outer wall of the internal dielectric tube 331 is
ionized, forming a cloud of plasma (atmospheric-pressure
non-equilibrium plasma).
[0109] The excitation light emitted from the atmospheric-pressure
non-equilibrium plasma travels through the space between the inner
wall of the external dielectric tube 311 and the outer wall of the
internal dielectric tube 331 to the region where the sample gas is
present, and ionizes the molecules (or atoms) of the sample
component in the sample gas. The thereby generated sample ions are
gathered to the ion-collecting electrode (i.e. the cylindrical
portion 323a of the flanged metallic tube 323) due to the electric
field created by the bias electrode (i.e. the exposed portion of
the metallic wire 322) located directly above the sample-gas exit
port, to be eventually detected as a current output. Consequently,
an ion current corresponding to the amount of generated sample
ions, i.e. an ion current corresponding to the amount of sample
component, is fed to the current amplifier 328. The current
amplifier 328 amplifies this current and provides it a detection
signal. In this manner, the present Ar-BID produces a detection
signal corresponding to the amount (concentration) of the sample
component contained in the introduced sample gas.
[0110] FIG. 3, the metallic wire 322 is made to function as the
bias electrode, and the flanged metallic tube 323 is made to
function as the ion-collecting electrode. Their functions may be
transposed. That is to say, the metallic wire 322 may be connected
to the current amplifier 328, and the flanged metallic tube 323 may
be connected to the bias DC power source 327. It is also possible
to replace the flanged metallic tube 323 with an element similar to
the cylindrical metallic electrode 122 or 123 provided in the
charge-collecting section in FIG. 1, and, make this element
function as the ion-collecting electrode or bias electrode.
[0111] The basic components and detecting operation of the Ar-BID
in the present embodiment are the same as those of the BID
described in Patent Literature 3. FIG. 7 shows the configuration of
the BID described in Patent Literature 3. In FIG. 7, the components
which are common to FIGS. 3 and 7 are denoted by numerals whose
last two digits are common to both figures.
[0112] The structural characteristics of the Ar-BID of the present
embodiment exist in the following points: the inner wall surface of
the external dielectric tube 311 is covered with the semiconductor
film 317; the length of the dielectric coverage area of the
metallic tube (ground electrode) 332 on the downstream side of the
lower end of the ring-shaped electrode (high-voltage electrode) 312
is made longer than the creeping discharge initiation distance
between the ring-shaped electrode 312 and the charge-collecting
section 320; and the length of the dielectric coverage area of the
metallic tube (ground electrode) 332 on the upstream side of the
upper end of the ring-shaped electrode (high-voltage electrode) 312
is made longer than the creeping discharge initiation distance
between the ring-shaped electrode 312 and the tube-line tip member
316.
[0113] When the semiconductor film 317 is formed on the inner wall
surface of the external dielectric tube 311 in the previously
described manner, the creeping discharge initiation distance
between the high-voltage electrode 312 and the tube-line tip member
316 as well as between the high-voltage electrode 312 and the
charge-collecting section (e.g. the lower-end area of the metallic
tube 332 which is not covered with the internal dielectric, tube
331, or the upper-end portion of the flanged metallic tube 323)
becomes shorter than when no semiconductor film 317 is formed. As a
result, the length of the dielectric coverage area of the metallic
tube 332 necessary for preventing the creeping discharge also
become shorter (than when no semiconductor film 317 formed).
Accordingly, in the Ar-BID according to the present embodiment, it
is possible to prevent an occurrence of the creeping discharge and
thereby improve the SN ratio while minimizing the increase in the
detector size.
[0114] Although the semiconductor film 317 may be any kind of
material, it is preferable to use a semiconductor which has a low
level of reactivity that makes the film resistant to sputtering and
which allows for easy formation of the film on the inner
circumferential surface of the external dielectric tube 311. For
example, a diamond-like carbon (DLC) film which can be formed by
plasma chemical vapor deposition (CVD) or a titanium oxide
(TiO.sub.2) film which can be formed by a sol-gel process can be
suitable used as the semiconductor film.
[0115] In the previously described example, the semiconductor film
317 is formed over the entire length of the external dielectric
tube 311. However, the minimum requirement is to form the
semiconductor film 317 on a least one of the following portions of
the external dielectric tube 311: the inner circumferential surface
of the area which covers the ring-shaped electrode 312 (this area
is hereinafter called the "ring-shaped electrode coverage area");
the inner circumferential surface of the area located downstream of
the ring-shaped electrode coverage area as well as upstream of the
charge-collecting section 320 (this area is hereinafter called the
"downstream-side area"); and the inner circumferential surface of
the area located upstream of the high-voltage electrode coverage
area as well as downstream of the tube-line tip member 316 (this
area is hereinafter called the "upstream-side area"). Providing the
semiconductor film 317 on the high-voltage electrode coverage area
and/or the downstream-side area shortens the creeping discharge
initiation distance between the high-voltage electrode 312 and the
charge-collecting section 320. Providing the semiconductor film 317
on the ring-shaped electrode coverage area and/or the upstream-side
area shortens the creeping discharge initiation distance between
the high-voltage electrode 312 and the tube-line tip member
316.
[0116] In the previously described example, the dielectric coverage
area of the metallic tube 332 is made longer than the creeping
discharge initiation distance on both the upstream side of the
upper end of the ring-shaped electrode 312 and the downstream side
of the lower end of the ring-shaped electrode 312. It is also
possible to make the dielectric coverage area of the metallic tube
332 on only one of the two sides, and particularly, only on the
downstream side of the lower end of the ring-shaped electrode,
longer than the creeping discharge initiation distance (between the
ring-shaped electrode 312 and the charge-collecting section 320).
In the ease of making only the dielectric coverage area of the
metallic tube 332 on the downstream side of the lower end of the
ring-shaped electrode 312 longer than the creeping discharge
initiation distance, the semiconductor film 317 is formed on the
inner circumferential surface of at least either the ring-shaped
electrode coverage area or the downstream-side area. In the case of
making only the dielectric coverage area of the metallic tube 332
on the upstream side of the upper end of the ring-shaped electrode
312 longer than the creeping discharge initiation distance, the
semiconductor film 317 is formed on the inner circumferential
surface of at least either the ring-shaped electrode coverage area
or the upstream-side area.
[0117] The length of the dielectric coverage area of the metallic
tube 332 on the downstream side of the lower end of the ring-shaped
electrode 312 is adjusted according to the kind, thickness and area
of the semiconductor film 317 on the ring-shaped electrode coverage
area and the downstream-side area of the external dielectric tube
311 as well as such parameters as the frequency and amplitude of
the low-frequency AC voltage, waveform of the power source,
material of the dielectric body, and property of gas so that no
creeping discharge will occur between the ring-shaped electrode 312
and the charge-collecting section 320. The length of the dielectric
coverage area on the upstream side of the upper end of the
ring-shaped electrode 312 is adjusted according to the kind,
thickness and area of the semiconductor film 317 on the ring-shaped
electrode coverage area and the upstream-side area as well as the
aforementioned parameters so that no creeping discharge will occur
between the ring-shaped electrode 312 and the tube-line tip member
316.
REFERENCE SIGNS LIST
[0118] 110, 210, 310 . . . Discharging Section [0119] 111, 211 . .
. Cylindrical Dielectric Tube [0120] 112, 211 . . . High-Voltage
Electrode [0121] 113, 213 . . . Upstream-Side Ground Electrode
[0122] 114, 214 . . . Downstream-Side Ground Electrode [0123] 115,
215, 315 . . . High AC Excitation Voltage Power Source [0124] 116,
216, 316 . . . Tube-Line Tip Member [0125] 117, 217, 317 . . .
Semiconductor Film [0126] 120, 320 . . . Charge-Collecting Section
[0127] 121, 221, 321 . . . Connection Member [0128] 122 . . . Bias
Electrode [0129] 123 . . . Collecting Electrode [0130] 126, 326 . .
. Sample Introduction Tube [0131] 127, 327 . . . Bias DC Power
Source [0132] 128, 328 . . . Current Amplifier [0133] 311 . . .
External Dielectric Tube [0134] 312 . . . Ring-Shaped Electrode
[0135] 334 . . . Electrode Structure
[0136] 331 . . . Internal Dielectric Tube
[0137] 332 . . . Metallic Tube
[0138] 333 . . . Insulating Tube
[0139] 322 . . . Metallic Wire [0140] 323 . . . Flanged Metallic
Tube
* * * * *